The susceptibility of AIDS patients to opportunistic infections is the distinguishing feature of the disease, and is ultimately responsible for its morbidity and mortality. Similar opportunistic infections plague other immunocompromised individuals, such as transplant patients and neonates. Thus, ubiquitous organisms that are ordinarily benign become debilitating pathogens in the immunosuppressed individual. Among the species that have become especially problematic in AIDS patients, and that are diagnosed increasingly in other populations, are the unique microsporidia, obligate intracellular parasites that are now recognized as common causes of a variety of severe, chronic conditions. Unfortunately, despite the prevalence of microsporidial infections, there are no proven effective drugs to combat these organisms or relieve the associated symptoms. This program is a collaboration between biologists at Tulane and George Washington, and medicinal chemists at Southern Research. The overall goal of the RO1 application is the development of novel, effective agents that will be useful for the treatment of microsporidial disease. To achieve this objective, the specific goals of this project are twofold: the design and synthesis of compounds that will effectively inhibit the growth and replication of microsporidia, with minimal host toxicity; and the development of advanced assays and models to evaluate these and other drugs, to help guide the synthetic program. Based on knowledge of microsporidial biochemistry and life cycle, we propose a structure- and mechanism-based program to develop agents that will undermine microsporidial replication through inhibition of mitosis and, possibly, polar tube extrusion. Specifically, we propose several series of heterocyclic tubulin ligands, related to 3-deazapurine and 1-deazapteridine, that have demonstrated good antimicrosporidial efficacy in vitro (see Section C). We also propose to adapt the synthesis of these compounds to more efficient combinatorial methods where appropriate, and to further develop our in vitro and in vivo assays and models.